Effects of high temperature on physico-mechanical ...

Engineering Geology 183 (2014) 127–136

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Effects of high temperature on physico-mechanical properties of Turkish natural building stones A. Ozguven a, Y. Ozcelik b,⁎ a b

General Directorate of Mineral Research and Exploration, Ankara, Turkey Department of Mining Engineering, Hacettepe University, Ankara, Turkey

a r t i c l e

i n f o

Article history: Received 26 December 2013 Received in revised form 25 June 2014 Accepted 7 October 2014 Available online 29 October 2014 Keywords: Natural building stones Temperature Thermal effect Fire Physico-mechanical properties Mineralogical–petrographical properties

a b s t r a c t It is known that fire and high temperatures cause degradation of natural building stones. There are many studies focused on the effect of high temperature on physico-mechanical properties of sandstone and granites while there are a few insufficient studies on limestones and marble. Almost all of the studies performed on limestone and marble are established at temperatures lower than 1000 °C and just focused on investigating some of the physico-mechanical properties of natural building stones and therefore some of the physical and mechanical properties of limestones and marbles exposed up to this temperature are not studied in detail. That condition cannot explain how the properties of marble and limestone change with high temperatures, which are widely used in many areas of our lives as building materials. The aim of this study is to investigate the changing of physico-mechanical properties of natural building materials including limestone and marbles exposed to different temperatures ranging from room temperature up to 1000 °C in the oven. For this purpose, samples were exposed to the heat separately starting from 200 °C, gradually 400, 600, 800 and 1000 °C and then some physico-mechanical properties of them and reference sample at room temperature were determined. The results obtained from the tests were analyzed in detail in terms of criticality of temperature degree, positive or negative effect of temperature rise, reusability of the building stone exposed to heat. As a conclusion of the study, important results are given in many aspects such as, usage areas of fire exposed natural stones, safety precautions at usage areas in addition to gained properties of natural stones due to temperature. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Natural stones are widely used in all areas of our lives. Exact properties of the natural stones must be known especially for constructing structures such as, buildings, bridges, and tunnels. Via this method safe and healthy living areas could be created. Besides knowing the most physico-mechanical properties of natural stones, it is also required to define how natural stones are affected by the heat which is an important factor. In many fields, such as energy, geology, civil engineering, the disposal of highly radioactive nuclear waste, the underground storage and mining of petroleum and natural gas, the development and utilization of geothermal resources, the safety of drainage and comprehensive utilization of coal seam gas and the post-disaster reconstruction of underground rocks engineering are all related to the strength and deformation characteristics of rocks under high temperatures (Liu and Xu, 2013).

⁎ Corresponding author. Tel.: +90 312 2976447; fax: +90 312 2992155. E-mail address: [email protected] (Y. Ozcelik).

http://dx.doi.org/10.1016/j.enggeo.2014.10.006 0013-7952/© 2014 Elsevier B.V. All rights reserved.

It is important to predict the behavior of natural stones, which are located around high temperature and heat sources, against the temperature to take necessary security measures with estimating the damage. The physical, mechanical, chemical and petrographical properties of rock, which are used as building materials, are important from the aspect of their uses in our daily life (Karaca et al., 2010). Marble is a material that is constantly used in building, either for structural (columns, floors, etc.) or decorative purposes (frieses, reliefs, statues, etc.). It is a noble material of particular beauty and easy manipulation, but it is susceptible to alteration by natural atmospheric agents or others resulting from urban and industrial activity (RodriguezGordillo and Saez-Perez, 2006). Rocks are composed of minerals, bounding matrix, and cracks and pores. The geometry and density of the cracks and pores are the main controlling parameters for the physical properties of rocks (Darot and Reuschlé, 2000; Yavuz et al., 2010). In engineering rock structures, temperature variation is one of the primary factors influencing the integrity and physical properties of rocks. It is responsible for the changes in microstructure of the rock by inducing new crack and micro crack development and so, for the increase in void space volume (Chaki et al., 2008).

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A. Ozguven, Y. Ozcelik / Engineering Geology 183 (2014) 127–136

Cracks, crusts and spalling were observed on the blocks following the fire hazard. One should note that the term crack refers to discontinuities formed by thermal gradients within the marble blocks during the fire (Koca et al., 2006). Fire disaster related changes in the petrological and petrophysical properties of the building materials can often lead to stability problem. It is important to establish the effect of fire and high temperature on the building structure, including natural stone. The walls of a building must necessarily attain a high temperature in a fire and suffer serious deterioration in strength and stability. Natural stone can be seriously affected in building fires, so it is useful to ascertain on which occasions the structure can be restored without replacement of stone and when it is necessary to include new stone material to maintain the structural integrity of the building (Hajpál, 2002). The calcinations of calcite from 800 °C may constitute the main effect generated by fire, as both the transformation from calcite to calcium hydroxide and the subsequent hydration of calcium hydroxide involve important volume changes that may alter the internal structure of the stone. CaCO3 + HEAT → CaO + CO2 ↑

CaO + H2O → Ca(OH)2 + HEAT These materials generally do not spall severely because the grains sit in a matrix and the stone can to an extent ‘absorb’ the stress produced by fire. Dense materials, such as granites or marble, experience physical breakdown due to the micro-cracking generated by the thermal expansion of minerals. The absence of a matrix, which in more porous materials absorbs the stresses generated by the expansion of mineral grains, increases the likelihood of mechanical breakdown. The very low porosity also favors this kind of disintegration due to the denser packing of minerals with different thermal and structural properties in the stone. It has been observed that, independent of the stone type, the lower the initial porosity the greater the porosity increase generated during fire—changes up to 13 times the initial porosity have been reported in building stones with low porosity. In addition to this, calcareous stones undergo severe processes of physical destruction in zones affected by fire above 800 °C due to the calcination of calcite (Gomez-Heras et al., 2006, 2009; Ozguven and Ozcelik, 2013). The most catastrophic change occurred in limestone cores, beginning to take place above 600 °C due to calcinations processes. Nine out of 10 Tardos limestone cores exploited during the 600 °C tests. Above 600 °C test, Süttő travertine samples were found undestroyed at the end of the test, but the samples crumbled after some hours exposed to air as a result of the volume increment produced by the reaction of CaO with air moisture to form Ca(OH)2 (portlandite). This process has been previously reported as a result of high temperature testing of stones containing calcite (Chakrabarti et al., 1996; Török and Hajpál, 2005; Gomez-Heras, 2006; Gomez-Heras et al., 2006). Researchers investigating the effects of different temperatures on natural stones have demonstrated different features of the natural stones. Some of these studies such as Blackwelder (1926) heating igneous rocks at different temperatures up to 880 °C as the presence of empirical observations or researchers such as Allison and Bristow (1999), Gomez-Heras et al. (2004), McCabe et al. (2007a) studied the weathering of rocks by the fire simulation. The most serious studies about the effect of temperature on the sandstones were made by Mahmutoglu (1998), Hajpál (2002), Hajpál and Török (2004), Gomez-Heras et al. (2004), McCabe et al. (2007a), McCabe et al. (2007b), Gomez-Heras et al. (2008), McCabe et al. (2010). Authors examined the changes occurred in sandstones by observational and experimental studies.

Darot and Reuschlé (2000), Reuschlé et al. (2006) and Chaki et al. (2008) investigated the micro-fractures and thermal damage in the studies done on granite samples which were exposed to temperatures up to 600 °C. Although the subject which the effects of temperature on the natural stones have been studied by different researchers, there are less number of articles that try to explain the changes of the stones by performing extensive physical and mechanical tests at very high temperatures. Studies have often tried to explain the behavior marble and limestone have shown against the heat, such as; investigation on degradation of historical buildings (Gomez-Heras et al., 2006; Koca et al., 2006; Rodriguez-Gordillo and Saez-Perez, 2006); or heat cycles with heating to 80–100 °C and cooling to −15–20 °C by examining a small number of samples (Malaga-Starzec et al., 2006; Yavuz and Topal, 2007; Lam dos Santos et al., 2011) or such as Yavuz et al. (2010) investigating some properties of heated samples up to 500 °C. Ozguven and Ozcelik (2013) investigated the effects of different degrees of temperature from room temperature up to 1000 °C (room temperature, 200, 400, 600, 800, 1000 °C) on marble and limestone by using the aspects of change in color and whiteness, polish reception, daily physical change, pH and temperature variations of the cooling solution which was prepared from the cooked samples at 800 °C and 1000 °C. They found that natural stones' structure becomes damaged and/or changes, brakes down, pours or cracks when heated above 800 °C. They also mentioned that natural stones that face this amount of heat under atmospheric conditions, crack, fragmentize, spall and disperse generally. The effect of temperature on marble was investigated by Liu and Xu (2013). They carried out dynamic mechanical experiments on marble under different temperatures and different impact loadings by using the high temperature split Hopkinson pressure bar (SHPB) experimental system. Their experimental results show that the stress–strain curves under impact loadings and different temperature show the same change trend below 800 °C. When temperature exceeds 800 °C, the densification stage prolongs, the curve moves towards right quickly, the slope decreases and the yielding stage extends evidently. The dynamic mechanical characteristics of marble take on obvious temperature effects. Brotóns et al. (2013) investigated the effect of the temperatures between 105 and 600 °C on the physical and mechanical properties of a porous rock namely calcarenite including porosity, ultrasonic wave propagation, uniaxial compressive strength, young modulus, Poisson's ratio and slake durability tests. Their tests were carried out under different conditions (i.e. air-cooled and water-cooled) in order to study the effect of fire off method. The results show that uniaxial compressive strength and elastic parameters (i.e. elastic modulus and Poisson's ratio), decrease as the temperature increases for the tested range of temperatures. Slake durability index also exhibits a reduction with temperature. Other physical properties, closely related with the mechanical properties of the stone, are porosity, attenuation and propagation velocity of ultrasonic waves in the material. All exhibit considerable changes with temperature. Sengun (2013) investigated the influence of temperatures ranging from 105 to 600 °C on the physical and mechanical properties of six carbonate rocks (two marbles, two low-porous limestones and two high-porous limestones). It was found that the values of bulk density, P wave velocity, uniaxial compressive strength and modulus of elasticity, Brazilian tensile strength and Shore hardness decreased to different extents, while apparent porosity increased under the influence of heat up to 600 °C. The results indicated a maximum decrease of 62–82% in modulus of elasticity and the least reduction of 1.2–2.7% in bulk density of carbonate rocks. Moreover, the uniaxial compressive strength, Brazilian tensile strength, P wave velocity and Shore hardness values decreased by 27–51%, 28–75%, 36–69% and 10–40%, respectively. Moreover, increase in apparent porosity values of tested rocks with very high porosity was the least, whereas the apparent porosity values of low-porous rocks increased up to ten times of initial value.


In all of the investigated studies, exposed temperatures are generally less than 1000 °C and therefore some of the physical and mechanical properties of limestones and marbles exposed up to this temperature are not studied in detail. For this reason, in this study the effect of high temperature on physical and mechanical properties of limestones and marbles having different textures and structural properties is investigated in detail. For this purpose, positive or negative impacts of the thermal effects on natural stones are examined. Expanding the use of natural stones with positive results, taking the necessary safety measurements against the adverse effects, and recommended kind of natural stones which are to be used in places with high risk of fire or traded with are explained at the end. 2. Materials and methods In the study, the aim is to determine the changes of the properties of the natural stones against the temperature, which are commonly available in Turkey, commercially producible, having different structures and textural characteristics. In this study, eight different samples (four of which are limestone and the others are marble) were used. Commercial names of samples used in the experiments with sample codes are given in Table 1. Firstly, samples were taken from natural stone quarries and drilling machine was used to take core samples, then they were cut with circular saw in the desired dimensions and the surfaces were smoothed. To determine the effect of temperature, samples which were non-

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exposed to heat were separated from the samples exposed to heat and were compared by the properties. In existing literature, three principal methods can be identified for the ‘laboratory’ investigation of fire effects. The most common method is to model the effects of the increased temperatures generated during fires within ovens. More recently, however, there has been some exploration of the use of real fires and laser-based techniques to replicate both temperature effects and associated chemical reactions. The use of ovens to simulate the heating generated by fire has the advantage of their availability and automatic function, as well as the high degree of standardization and replication that can be obtained in such tests. In addition, the wealth of results in the literature obtained with this technique guarantee a database for comparison of new results (Allison and Bristow, 1999; Hajpál and Török, 2004; Gomez-Heras et al., 2009. For this reason, oven was preferred in this study. To explain the effect of temperatures, different degrees of temperature were selected. The purpose of this operation is to clearly show what kind of changes occurs at different temperatures. The highest temperature used in the study was determined as 1000 °C which is the highest temperature expected in a fire incident and for the alteration of natural stones. Samples were exposed to the heat separately starting from 200 °C, gradually 400, 600, 800 and 1000 °C and then compared with reference sample at room temperature according to the degree of temperature. By using Protherm PLF 130/25 model oven, samples were heated considering the oven conditions with a heating rate of 5 °C per minute. Exposing the natural stones to the required temperatures was

Table 1 Petrographic descriptions and microphotographs of the samples. Sample code

Sample name

Petrographic descriptions

HP

Hazar Pink

Sparitic limestone. Bioclast, calcite and abundantly fossils are present.

DB

Daisy Beige

Biosparitic limestone. Moderately crystalline calcite and small amount of opaque minerals are present.

SB

Sivrihisar Beige

Sparitic limestone. Moderate crystalline calcite and a small amount of recrystallized thinney calcite veins and opaque minerals are present.

BB

Burdur Beige

Sparitic limestone. Moderate crystalline calcite and a small amount of recrystallized thinney calcite veins and opaque minerals are present.

AW

Afyon White

Marble. Granoblastic texture. Moderate crystalline calcite and a small amount of recrystalised thinney calcite veins and opaque minerals are present.

AG

Afyon Gray

Marble. Granoblastic texture. The calcites with pressure twining are present.

AT

Afyon Tigerskin

Marble. Granoblastic texture. Coarse–moderate crystalline calcite and small amount of opaque minerals and quartz are present.

MM

Mugla Milas

Marble. Granoblastic texture. Calcite and small amount of muscovite, quartz, and opaque minerals are present

Microphotographs of the samples

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intended with this process. After reaching the desired temperature, natural stones were kept for 1 h to inflict enough exposure to the temperature effect. Natural stones were stored in the oven for 1 h in order to let the physical and mechanical changes take place at the required temperatures. Natural stones exposed to high temperatures were cooled down to room temperature inside the oven to avoid thermal shock. Thin sections of the natural stone samples were prepared and then were examined under a polarized microscope to determine the textural features of each sample. The petrographic descriptions and microphotographs of the samples were determined from these thin-sections and the results are given in Table 1. To introduce the different aspects of the physico-mechanical changes of marble and limestone depending on the temperature, samples which were not exposed to temperatures and the ones exposed to different temperatures were prepared and tests were carried out under the conditions specified. The physico-mechanical tests include density (D) (ASTM D5550-06, 2006), bulk density (BD), ratio of fullness (RF), porosity (P), apparent porosity (AP), freeze loss (FL), freeze–thaw resistance(FTR) (TS 699, 1987), Mohs hardness (TS 6809, 1989), water absorption at atmospheric pressure (WA) (TS EN, 13755, 2009), water absorption coefficient by capillarity (WACC) (TS EN, 1925, 2000), uniaxial compressive strength (UCS)(EN, 1926, 2006), indirect tensile strength (TS) (TS 7654, 1989), abrasion resistance (AR) (EN, 14157, 2004). Some physico-mechanical properties of all samples before exposing to the high temperatures are given in Table 2. 3. Results and discussion 3.1. Density

Fig. 1. Density change related to temperature: a) limestones, b) marbles.

Density values measured from the samples which were heated from room temperature up to 1000 °C gradually and the results are given in Fig. 1. When Fig. 1 was analyzed, the following results were obtained. For limestones up to 800 °C and for marbles up to 600 °C, density values do not change significantly with the temperature. This situation was discussed by Ferrero and Marini (2001) in their study. They stated that the real density does not show significant changes after heating. This hypothesis appears realistic since the rocks are basically made up of pure calcium carbonate with small percentages of magnesium, and the heating temperatures are much lower than the carbonate dissociation temperature (900 °C). With increasing temperature, a slight decrease and increase occurs in density values while the values mostly remain within the same trend. As the temperature increased, structural deformations took place therefore changing the real density values. For two types of stone sudden drop begins at 800 °C. It is remarkable that density values do not change significantly for the limestone samples DB and HP and marble samples AT. The limestone sample SB's and the marble sample MM's densities are drastically reduced. The reason for this is considered to be the easy decomposition of SB and MM after

the exposure to high heat. Ozguven and Ozcelik (2013) also stated that SB and MM were the first to decompose when exposed to high heat in their study observing daily changes in the natural stones exposed to high heat. 3.2. Bulk density Bulk density values measured from the samples which were heated from room temperature up to 1000 °C gradually and the results are given in Fig. 2. When Fig. 2 was analyzed, the following results were obtained. Bulk density decreases dramatically with the increase in temperature. Reduction in bulk density increases after 400 °C. This is caused by the capillary cracks that occur in natural stones, expansion and materials left from the structure. Changes in the bulk density of limestones, decreased with the same slope of the different varieties of limestones, except HP. HP sample is separated from the other samples with these properties, as well as density. In the samples, less change with temperature is realized. Bulk density values decrease considerably at

Table 2 Some physico-mechanical properties of the samples used in the study. Tests D BD RF P AP H WA WACC UCS TS AR FL FTR

Units 3

kg/m kg/m3 % % % Mohs % g/m2 s0.5 MPa MPa cm3/50 cm2 % MPa

HP

DB

SB

BB

AW

AG

AT

MM

2737.9 2636.1 96.3 3.7 2.4 4.0 0.9 1.575 149.2 8.52 11.7 0.03 122.7

2723.9 2663.3 97.8 2.2 1.7 4.0 0.6 0.985 187.4 10.35 10.3 0.03 191.1

2722.2 2695.2 99.0 1.0 0.6 3.5 0.2 0.400 137.2 10.22 13.7 0.02 134.7

2748.3 2683.2 97.6 2.4 2.0 4.0 0.7 0.888 139.3 9.16 12.3 0.01 183.8

2716.2 2711.1 99.8 0.2 0.2 3.0 0.1 0.187 83.3 7.86 23.6 0.01 85.1

2716.2 2712.8 99.9 0.1 0.2 3.0 0.1 0.151 80.4 9.60 24.6 0.01 92.0

2728.5 2710.3 99.3 0.7 0.3 3.0 0.1 0.142 70.3 6.89 23.6 0.01 74.2

2725.9 2707.3 99.3 0.7 0.4 3.0 0.1 0.232 61.2 5.04 23.0 0.01 77.5


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Fig. 2. The effect of temperature on the weight of unit volume: a) limestones, b) marbles. Fig. 3. The effect of temperature on occupancy rate: a) limestones, b) marbles.

800 °C. Decrease rate in bulk density of marbles increases after 200 °C. Decreasing bulk density values of the samples with increasing temperature are observed for almost all the marble samples. As the heat increased, increases in bulk density were also observed due to increase in void space of the rock. This case does not draw an analogy with the changes happen in the density. This phenomenon occurs because mineral structure and void space of the rock are affected by heat changes for different rates. Therefore changes in density due to heat are limited.

Increase of porosity in the marbles is more than in the limestones. Porosity of HP sample from limestones increases very slightly while the other limestones increase more. Especially at temperatures above

3.3. Ratio of fullness Ratio of fullness values measured from the samples which were heated from room temperature up to 1000 °C gradually and the results are given in Fig. 3. When Fig. 3 is analyzed, the following results were obtained. Ratio of fullness of natural stones decreases depending on the effect of temperature rise. Critical temperature is determined as 400 °C, which changes the ratio of fullness. Ratio of fullness values of HP sample in limestones has the minimum change in comparison to other samples, while other samples' ratio of fullness values decreases. Ratio of fullness values decreases with increasing temperature in marbles. AW and AG samples seem to be the most reduced samples. 3.4. Porosity The strength of a material decreases with increasing porosity but is also related to pore size, pore shape and spatial distribution. Pores may occur within the grains or in the grain boundaries. A comprehensive study of porosity can provide valuable information in order to determine whether a given type of marble is susceptible to thermal stresses or not (Malaga-Starzec et al., 2006). Porosity values measured from the samples which were heated from room temperature up to 1000 °C gradually and the results given are in Fig. 4. When Fig. 4 was analyzed following results were obtained. Porosity values of natural stones increase with the increase in temperature.

Fig. 4. The effect of temperature on porosity: a) limestones, b) marbles.

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400 °C, porosity increases more. Porosity values of marble samples increase when the temperature increases. Porosity increases faster for samples AW and MM above 200 °C, for AG and AT above 400 °C. Gomez-Heras et al. (2006) obtained similar results in their study. They mentioned that the porosity increased in all the studied stones present when they were heated to simulate the effect of fire. The rate of porosity change is influenced by the “compactness” of stone. The compact stones show more dramatic change in porosity at elevated temperatures than the less cemented ones. Mechanical property changes of natural stones caused by increase of porosity due to temperature are features to be noted. Increase in porosity of samples explains the decrease of compressive strength very well. This also explains the decrease in the strength of the lattice structure. 3.5. Apparent porosity Apparent porosity values measured from the samples which were heated from room temperature up to 1000 °C gradually and the results are given in Fig. 5. When Fig. 5 was analyzed, the following results were obtained. For the limestones and marbles, almost the same tendency is seen depending on the temperature. There are visible porosity increases seen at 400 °C for limestones and at 200 °C and above for marbles. This increase was slow at the beginning while it got faster above 600 °C both for limestones and marbles. HP sample from limestones and MM sample from marbles are the most affected natural stones by temperature. 3.6. Hardness Fig. 6. The effect of temperature on hardness: a) limestones, b) marbles.

Hardness values measured from the samples which were heated from room temperature up to 1000 °C gradually and the results are given in Fig. 6. When Fig. 6 was analyzed, the following results were obtained. Hardness values of the temperature exposed natural stones do

not change significantly with the temperature change. There is a decrease about 1 Mohs between the hardness value of the sample in the room temperature and the hardness value of the sample heated to 1000 °C. Hardness values decrease for all natural stones exposed to temperature. It is observed that there are some differences caused by the composition and structure differences of limestones. It is seen that the hardness values change at 200 °C for SB and DB and at 400 °C for BB and HP samples. In all of the natural stone samples used in the study, the DB sample has the highest hardness loss. Hardness of the DB dropped from 4 Mohs to 2 Mohs. For the marble samples, except MM, hardness change is observed with the same tendency by the effect of temperature. Hardness of MM sample starts to decrease at 400 °C while others start to decrease at 600 °C. Hardness values of the samples are at the lowest level at 800 °C and 1000 °C while fractures, cracks and distortion are formed is a result of temperature that must be taken into consideration.

3.7. Water absorption at atmospheric pressure Water absorption values at atmospheric pressure were measured from the samples which were heated from room temperature up to 1000 °C gradually and the results are given in Fig. 7. When Fig. 7 was analyzed, the following results were obtained. Water absorption values increase in direct proportion with exposure to temperature of natural stones. Initially, this ratio is small but as the temperature increases a significant increase is observed in the rate of water absorption. Temperature variation and water absorption are parallel characteristics of limestones. Change in the water absorption rate increased almost in the same amount. HP sample showed the same trend with other limestones until 600 °C while the rate of water absorption further increased at 800 °C. At marbles, changes in the water absorption rate are parallel to each other. While others absorb almost the same proportions of water, MM sample is the most absorbing one. Fig. 5. Apparent porosity change related to temperature: a) limestones, b) marbles.


Fig. 7. The effect of temperature on water absorption rate: a) limestones, b) marbles.

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Fig. 8. The effect of temperature on capillary water absorption coefficient: a) limestones, b) marbles.

3.8. Water absorption coefficient by capillarity Water absorption coefficient by capillarity values measured from the samples which were heated from room temperature up to 1000 °C gradually and the results are given in Fig. 8. When Fig. 8 was analyzed, the following results were obtained. One of the most important properties is the determination of the capillary water absorption coefficient to investigate the effect of high temperatures on the natural stones. This feature enables to determine the capillary cracks of natural stones physically. It is seen that the temperature change directly affects the capillary water absorption. Capillary water absorption increases with the increase of temperature, as seen on the graphs. Very smooth graphs obtained in this experiment explain the effect of temperature well. Capillary water absorption property of limestones does not change significantly up to 200 °C while capillary water absorption rates increases rapidly above this temperature. The growth rate increases at 600 °C. Capillary water absorption ratio of marbles increases a little while the capillary water absorbs more above this temperature. MM sample is separated from the other marbles by absorbing the maximum amount of water. 3.9. Uniaxial compressive strength Uniaxial compressive strength values measured from the samples which were heated from room temperature up to 1000 °C gradually and the results are given in Fig. 9. When Fig. 9 was analyzed, the following results were obtained. Strength of the natural stones is adversely affected by the temperature changes. Limestones keep some of their strength up to 400 °C. This is because of limestone's transformation into lime after this temperature. After 400 °C, compressive strength values rapidly decrease. Relative increase occurs in marble up to 200 °C, while the strength decreases with increase in temperature. This situation fits the study of Koca et al. (2006). In the study of Koca et al. (2006), unlike the general trend, increase in uniaxial compressive strength is seen at 200 and 250 °C. The critical temperature for both

Fig. 9. Compressive strength change related to temperature: a) limestones, b) marbles.

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natural stone types is 600 °C. Pressure resistance drops suddenly when natural stones are exposed above this temperature. MM sample loses its strength and even become dispersed at 800 °C and above it. For this reason experiments at 1000 °C was not possible. Both for limestones and marbles the pressure values decrease four times at 1000 °C. Liu and Xu (2013) had similar results and they stated that before 800 °C, with temperature rising, the dynamic failure modes of rock under the same impact velocity gradually become more and more tempestuous. Although the strength of marble remarkably reduces, the dynamic failure modes mutate at 800 °C. The amount of fragments decreases and the size increases obviously. When temperature continues to increase, reaching 1000 °C, the specimen almost loses its carrying capacity, presenting powder failure. At the same time, they also revealed that the dynamic failure modes have no necessary relationship with strength.

3.10. Tensile strength Tensile strength values measured from the samples which were heated from room temperature up to 1000 °C gradually and the results are given in Fig. 10. When Fig. 10 was analyzed, the following results were obtained. At temperature exposed natural stones, tensile strength decreased with increasing temperature. At limestones, up to 800 °C temperature increase/decrease was observed. BB and DB samples after 400 °C and HP and SB samples after 600 °C tend to have a downward trend in tensile strength. Tensile strength of marbles has been changing parallel to each other. Tensile strength decreases with the increase in temperature. Sudden changes were not observed at any of the temperatures. Places that natural stones are used have gained great importance due to tensile strength's reduction with rise of temperature. Building stones must be used considering the security issues at places which are likely to reach high temperatures.

Fig. 10. Temperature effect on indirect tensile strength: a) limestones, b)marbles.

3.11. Abrasion resistance Abrasion resistance values measured from the samples which were heated from room temperature up to 1000 °C gradually and the results are given in Fig. 11. When Fig. 11 was analyzed, the following results were obtained. It can be said that all natural stone samples have the same trend as the wear characteristics when abrasion resistance is analyzed. While there was no significant change in abrasion resistance up to a significant temperature, abrasion of the samples increased significantly at the critical temperature. It is observed that the wear resistances of the marble samples and limestone samples are close to each other respectively. It is observed that limestones have approximately the same wear resistance up to 800 °C. All samples, except MM, have almost the same resistance to abrasion up to 600 °C. The critical temperature of limestones is 800 °C. After 800 °C, abrasion increases rapidly. The critical temperature of marbles is 600 °C. After 600 °C, abrasion increases rapidly. After the critical temperatures, abrasion values suddenly increased up to four times for both types of natural stones. Just because of MM sample's grain size is greater, abrasion resistance experiments deteriorate rapidly with increasing temperature. 3.12. Freeze loss Freeze-loss values measured from the samples which were heated from room temperature up to 1000 °C gradually and the results are given in Fig. 12. When Fig. 12 was analyzed, the following results were obtained. No freeze loss was observed up to 600 °C at all samples, except MM. After 600 °C frost mass losses occur in different proportions. For sample MM, frost mass loss begins at 200 °C. The most affected sample from the freeze and thaw experiment by the effect of temperature is MM. All of the limestones show different behaviors at temperatures above 600 °C. The most interesting of these behaviors is the increase in the mass after frost occurs at BB sample. After the freeze–thaw test, different rates of mass loss also occur at marbles

Fig. 11. Abrasion resistance change related to temperature: a) limestones, b) marbles.


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Fig. 13. The effect of temperature on frost mass loss: a) limestones, b) marbles. Fig. 12. The effect of temperature on loss of freeze: a) limestones, b) marbles.

above 600 °C. If natural stones, which exposed to fire and high temperatures, didn't reach 600 °C due to the effect of seasonal temperature changes, it can be stated that there would not be any problems considering the freeze–thaw test results. 3.13. Freeze–thaw resistance Freeze–thaw resistance values measured from the samples which were heated from room temperature up to 1000 °C gradually and the results are given in Fig. 13. When Fig. 13 was analyzed, the following results were obtained. Compressive strength values for samples after freeze–thaw experiment and samples which are not subjected to freeze–thaw experiments are almost the same. The critical temperature of limestones is 400 °C. Up to 400 °C, decreasing/increasing values were observed and after this temperature compressive strength of all the samples decreases. For the marbles, generally compressive strength decreases at similar rates for each sample with the increase in temperature. At any temperature sudden drop/increases were observed. 4. Conclusions The following results were obtained from the studies done with the samples which have different characteristics of limestone and marble under the influence of high temperature and fire to determine the changes in physico-mechanical properties against temperature to natural building stones. The temperature above 800 °C, calcite turns into portlandite. This conversion leads to changing of structural and textural properties of natural building stone. The results are given by considering this conversion. • Density does not affect the temperature significantly. • Increase in the temperature causes decrease in the weight of bulk density caused. By analyzing the densities of all natural stones, the effect of temperature changes with the same tendency was determined.

• Increase in ratio of fullness of natural stones by decrease of temperature was determined. One of the most important features describing the deterioration of physico-mechanical properties of rocks is decrease in occupancy rate. • It is determined that increase in the temperature increases the porosity of the natural stones. • When the temperature increases, apparent porosity values also increase. This situation is caused by capillary cracks occurring due to heat, materials from the structure, etc. • The effect of temperature on hardness is limited. It is determined that there is not a huge change on hardness of the samples even with a rise of temperature. • Considerable increases in the amount of water absorbed have been identified on natural stones under the influence of high temperatures. These increase rates for all kinds of marble and limestone are approximately parallel to each other. • Increase in the amount of absorbed water by increasing temperature can trigger the structural demolitions. Therefore, safety measurements should be taken while exposing with water when a fire has been detected. • It is determined that rising temperature produce capillary cracks and capillary cracks cause increase in the capillary water absorption coefficient on the natural stones. • Temperature changes for natural stones are affecting the compressive strength values. Especially, strength values of the samples with 600 °C and above are relatively low. Therefore, using the natural stones as a carrier in buildings is not convenient, considering the building would possibly reach these temperatures. • Tensile strength of the samples decreases with increasing temperature. For the limestones, this is observed in an irregular shape at different temperatures with decrease/increase. For the marbles, strength is regularly decreases depending on the increase of temperature. • Usage areas of the natural stones exposed to fire or high temperatures are very important depending on the level of temperature.

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